(1) Field of the Invention
The present invention generally relates to ferroelectric materials and devices. More particularly, this invention relates to circuit devices comprising two or more ferroelectric elements with polarization gradients, wherein the elements can be constructed on a common platform together with a number of active or passive components, such as transistors, resistors, capacitors and inductors.
(2) Description of the Related Art
Up until quite recently, the primary focus of ferroelectric thin film research has been to reliably reproduce the properties of bulk materials in thin film form. However, the study of ferroelectric thin film materials only in terms of their bulk properties fails to capture the full richness of these materials, much as a limited study of the bulk properties of semiconductor materials would fail to anticipate their use as compositionally-doped structures such as diodes and transistors. It is with this realization that the growth and analysis of chemically and structurally non-uniform ferroelectric materials have been undertaken. A variety of ferroelectric material systems have been considered, including but not limited to barium strontium titanate (Ba1—xSrxTiO3; BST), potassium tantalum niobate (KTa1—xNbxO3; KTN), lead calcium titanate (Pb1—xCaxTiO3; PCT), lead zirconate titanate (PbZrxTiyO3; PZN), lead strontium titanate (Pb1—xSrxTiO3; PST), and lead lanthanum titanate (Pb1—xLaxTiO3; PLT).
When subjected to an applied electric field, a rapid and major polarizing effect occurs in a ferroelectric material, and remnant polarization is observed both in the presence and after removal of the field. As reported in commonly-assigned U.S. Pat. Nos. 5,272,341, 5,386,120 and 5,448,067 to Micheli et al., a ferroelectric material having a graded composition exhibits a new hysteresis effect, manifested by a hysteresis loop translation along the polarization axis in a polarization (P) versus electric field (E) plot. Micheli et al. report that a significant and unexpected pseudo-pyroelectric response is exhibited by such ferroelectric heterostructures (what are termed herein graded ferroelectric devices, or GFD's), as a result of a gradient in dipole moment (charge) density (and therefore, a polarization gradient) normal to the growth surface. Much like semiconductor diode junctions, which produce asymmetric current-voltage characteristics as a result of “built in” potentials across chemically graded junction regions, Micheli et al. showed that polarization-graded ferroelectrics exhibit shifted charge-voltage hysteresis (“up” and “down”) loops which are also attributed to “built in” potentials. Unlike semiconductor junctions, however, whose potentials arise from a diffusion of free charge across chemically graded junctions, the intrinsic potentials in graded ferroelectrics are due to gradients in bound charge or dipole moment density. These potentials may be shaped in much the same manner as advanced forms of semiconductor devices (such as quantum well structures) by suitably tailoring the gradients in permanent dipole charge density.
To understand the behavior of GFD's, one can consider the properties of a ferroelectric with a linearly graded polarization P (bound charge dipole-moment per unit volume) normal to the growth surface (the substrate/film interface arbitrarily designated z=0), and write P(z)=D(z)−εoE(z). Here, εo is the permittivity of free space, D the electric displacement, and E the electric field. For ferroelectrics, at any fixed z, P(z) changes abruptly from zero (P(z)=0 when E=0) at a Curie temperature Tc(z) and increases or remains essentially constant with decreasing temperature below the transition temperature, as shown schematically in FIG. 1. Below the lowest Curie temperature of a GFD, the free-energy diagram for the structure would appear as in FIG. 2 where, with depth, there is a series of double-well structures characteristic of the two polarization states. Unlike a structure consisting of discrete laminated layers of ferroelectric, however, each well is skewed to lower energy (in this example with decreasing z) because the gradient in polarization is a coεntinuous function of depth.
FIGS. 3 and 4 schematically represent, respectively, a polarization-graded ferroelectric film connected to an alternating voltage source, and the resulting dipole-moment profile through the material. The dipole-moment gradient through the ferroelectric film is normal to the film thickness (parallel to the z-axis of the material), and alternating voltage is applied across the thickness of the film. The degree of dipole alignment and polarization strength are functions of many parameters, including temperature, pressure (stress/strain), composition, and applied external electric field (E). As such, the polarization gradient may be achieved by any number of methods, such as (1) the imposition of a temperature gradient through the film thickness, (2) a compositional gradient normal to the film growth surface, and (3) a stress/strain gradient through the film thickness. As reported in the above-noted patents to Micheli et al., normal hysteresis for a uniformly polarized ferroelectric film is seen in a polarization versus field (P vs. E) plot, which shows how the polarization of a ferroelectric material may be switched between two states. However, as also reported in Micheli et al., the presence of a polarization gradient through the ferroelectric film of FIG. 3 alters the usual symmetry found in a Q vs. V plot, as shown in FIG. 5 (where Q is meant to mean dipole-moment per area). In the polarization-graded ferroelectric film, there is an internal electric field generated by the polarization gradient. This internal electric field occurs across both the ferroelectric and the sampling capacitor found in typical measurement circuitry, such as the modified Sawyer-Tower circuit, where the two voltages are equal but in opposition. This non-zero internal electric field gives rise to a measurable potential V, manifested as a shift in static (dc) voltage when an alternating (ac) voltage (e.g., sine wave) is applied to the film, indicating that there is a significant voltage developed by a gradient in the dipole-moment per unit area.
For small gradients in composition, temperature or stress, the dc voltage shift V may be written:V=K∇q  (Eq. 1)with K a constant which may depend upon temperature, and ∇q may be: ∇c, a compositional gradient; ∇T, a temperature gradient; ∇σ, a stress gradient. As such, the graded dipole moment of the film produces a charge increase on the opposing surfaces of the film when a constant ac voltage is applied to the film surfaces and additional energy (e.g., thermal, radiant, mechanical, or electrical) is imparted to the film, such that an internal electric field produces a potential that varies in response to the additional energy and causes a translation of the hysteresis loop along the charge separation axis (Q) of the charge separation versus voltage plot (Q vs. V), as represented in FIG. 5. The degree of translation along the charge separation axis is a function of the additional energy imparted to the film (e.g., temperature, pressure, applied field, etc.), and the observed translation of the hysteresis loop is a dc offset that is a measure of the additional energy. Both forward-polarized (“up”) and reversed-polarized (“down”) GFD's can be fabricated by suitably grading the polarization, such as by grading the barium to strontium ratio in a barium strontium titanate (Ba1−xSrxTiO3) film. Similar results have likewise been observed in a variety of other material systems, including potassium tantalum niobates, lead calcium titanate, lead zirconate titanate, and lead lanthanum titanate GFD's.
In view of the above, while a semiconductor junction has an asymmetric current-voltage characteristic (free charge flow in the presence of an applied field), FIG. 5 shows that a polarization-graded ferroelectric structure has a displaced charge-voltage characteristic (net bound charge alignment in the presence of an applied periodic field). Trans-resistive (“transistor”) semiconductor devices are formed by modulating the free charge carrier density across an n-p-n or p-n-p junction transistor to create a device which modulates free charge flow and is capable of power gain. In semiconductor transistors, power gain is achieved when a small signal current is injected into the base region of a transistor and is amplified sufficiently to drive a load by means of transistor action. The power sources for transistors are the dc supply voltages. In contrast, a GFD can be termed a transcapacitive (“transpacitor”) charge storage/voltage generating device, represented in FIG. 6, wherein the internal potential (V of Equation 1) is altered by a modulating energy flux to the device, such as heat or strain energy, resulting in the dynamic property represented in FIG. 5.
For a GFD, V is also a function of the peak excitation voltage of the ac power source. Therefore, the energy transferred to a capacitive load, C, (approximately ½CV2 when the load capacitance is large compared to the “capacitance” of the GFD) is a strong function of the small modulating energy flux. This phenomenon may be used to great effect to enhance the pyroelectric property of ferroelectric materials. For a GFD structure one can define an effective or “pseudo” pyroelectric coefficient as:peff=∂(ΔD)/∂T)Eac  (2)where ΔD is the translation of the hysteresis loop (up or down) at a fixed temperature, and Eac denotes a periodic bias field excitation whose magnitude is held fixed. At constant Eac, peff represents (again, proportional to a sourced energy) the change in the relatively large area swept out by the translated hysteresis loop with temperature due to an injected external heat flux. This “transpacitor” action greatly amplifies the normal pyroelectric coefficient in a GFD, typically yielding (for graded ferroelectric devices) pyroelectric coefficients on the order of 1–10 μC/cm2·° C., nearly a ten thousand fold increase in sensitivity over that of a non-GFD device.
While the properties of a few rudimentary GFD devices have been characterized and analyzed theoretically, the potential of this technology has yet to be adequately explored. Importantly, there presently does not exist an analogous hybrid model, similar to those developed for transistors, which can guide the designer or theoretician in describing transpacitor behavior. In addition, a robust fundamental analysis of such structures has not been undertaken, thus leaving ample opportunity for the discovery of new effects related to GFD's. Therefore, while research thus far is quite compelling from a basic science point of view, the real applicability of the technology resides in the ability to form electronic devices that make use of the unique properties of a GFD.